1. Field of the Invention
The invention relates generally to processors which include a data bus for communicating large amounts of data, such as video or graphics data, to a device on the bus, such as an external device. In particular, the invention concerns providing a dedicated bus that avoids the need for transmitting such information through other system buses.
2. Related Art
FIG. 1 is a block diagram of a conventional system. A central processing unit (CPU) 1 is connected through a data communication bus 3 to an interface 5 and a high level L2 cache memory 7. L2 cache 7 communicates with another cache 8 over link 11. The L2 cache 7 is connected to a memory control unit 9. Bridge 17 links the system to PCI bus 19. The PCI bus 19 has various elements connected thereto. These could include a double or quad speed CD ROM 21, a graphics controller 23 and possibly a digital signal processor (DSP) 25. Graphics controller 23 is also connected to memory 27 and is used to drive display 29.
In a conventional system as shown in FIG. 1, compressed video is supplied from a video source, such as CD ROM 21, under control of CPU 1, onto PCI bus 19. DSP 25, under control of CPU 1, processes the compressed video to create decompressed video for delivery to graphics control unit 23 for display of a corresponding image on display 29.
Recent advances in video processing have improved the conventional system of FIG. 1 to yield a system as shown in FIG. 2. DSP 25 is no longer connected to PCI bus 19, thus reducing the hardware and real estate needed to implement the system. Instead, digital signal processing is accomplished within CPU 1.
The digital signal processing in CPU 1 can take either of two forms. A first form is the incorporation of a conventional DSP, such as DSP 25, onto the microprocessor chip comprising CPU 1. A second form is the use of processing wherein the activities previously accomplished by a DSP are accomplished by the CPU according to software. In either form, the incorporation of the DSP activities, such as the task of decompressing compressed video to produce decompressed video, into CPU 1, can lead to drawbacks. As described below, these drawbacks are addressed by the invention.
For a conventional display 29 containing 1024 by 768 pixels, production of one high color image requiring 2 bytes per pixel requires about 1.6 megabytes of data. At 8 bits per byte such an image requires about 12.5 M bits. To produce a full motion video image, a frame rate of 30 frames per second is required. Thus, production of a full motion 16 bit full color video image on display 29 requires about 48 megabytes of data per second.
In the system of FIG. 2, CPU 1 executes the decompression algorithm, and the decompressed video is routed through bridge 17 to PCI bus 19. PCI bus 19 has a peak bandwidth of 133 megabytes, with about 50 megabytes usable. As noted above, a high color image requires 48 megabytes per second of decompressed video. A PCI bus has a peak capability of about 132 M bytes per second. However, this rate is not sustainable because bus overhead reduces the useable bus bandwidth to about 50 M bytes/sec. Since PCI bus 19 has a usable capability of only about 50 megabytes per second, production of decompressed video routed to graphics controller 23 consumes virtually all of the capability of PCI bus 19, thereby leaving little bandwidth for use by other elements such as CD ROM 21 and DSP 25. New 64 bit, 66 MHz PCI bus configurations are faster, but have other drawbacks. Such drawbacks include electromagnetic interference (EMI), increased cost and limits on the number of available slots per bridge, thereby requiring more bridges and further driving costs up.
In the conventional system of FIG. 1, the close physical proximity of DSP 25 to graphics controller 23 minimized the negative impact of the bus dominance by the decompressed video from DSP 25 to graphics controller 23. However, in systems such as that of FIG. 2, wherein the digital signal processing is occurring in CPU 1, this bus dominance leads to degradation of the video image.
For example, since more than two megabytes of bandwidth are needed for CD ROM 21 to provide the compressed video to CPU 1, the result is that 48 megabytes of bandwidth on the PCI bus are not always available for the delivery of decompressed video to graphics controller 23. When the decompressed video is not available to graphics controller 23, one or more video frames may be dropped. When the frame rate falls below 30 frames per second, the resulting video image may appear degraded. To compensate for this situation, designers have opted to use only a portion of video display 29, such as a window, to show full motion video. By using less (fewer pixels) of the display, a smaller bandwidth is required for the decompressed video, the frame rate can be maintained, and sufficient bus capacity exists to allow other devices to communicate via PCI bus 19. However, the constraint of using only a portion of the available display for full motion video is limiting.